1. Field of the Invention
The present invention relates to a new process for partial electroless plating on a plastic substrate. This process requires that only the part for plating be coated with a coating compound which forms a film of resin composition with a special structure so that electroless plating film is deposited only on thus formed coating film. Hence, this process does not need any corrosion step. The present invention can be applied to any field in which electroless plating is used; however, it is advantageously applied to electromagnetic wave shielding particularly that of heat-resistant resin and printed circuit boards.
2. Description of the Related Art
Electroless plating is performed on plastic moldings which are electrically non-conductive so that they are made conductive and receptive to electroplating. It is preceded by pretreatment to corrode the surface of molding to be plated, thereby forming minute etch pits in the surface layer. In the subsequent electroless plating step, these etch pits produce the anchoring effect on metal deposited on their inside wall, so that the plated layer is bonded to the surface of the substrate. The etching step may be preceded by pretreatment such as swelling by a solvent which facilitates etching in the case of such resins as propylene and polyether imide which are difficult to etch.
A process for electroless plating on plastics, which employs an adhesive for electroless plating, is disclosed in Japanese Patent Laid-open Nos. 8281/1990 and 22081/1991. It is designed for production of printed circuit boards. The adhesive used in this process has a composition which permits corrosion more readily than the substrate. Prior to electroless plating, this adhesive is applied to the substrate and then corroded so that etch pits are formed in the surface layer of the adhesive. The resulting etch pits produce the anchoring effect for the coating film to adhere to the substrate, as mentioned above.
If electroless plating is to be performed partially on a substrate, it is necessary to apply a masking paint to the part which does not need plating. This masking paint should be resistant to corrosion and should prevent the deposition of plating film. With the masking paint partially applied, the substrate undergoes corrosion and electroless plating. The masking paint is left unremoved after electroless plating. This is a common practice in decorative plating.
The conventional technology of electromagnetic wave shielding, to which the present invention is applicable, includes coating with an electrically conducting paint, thermal spraying of zinc, and lamination or sandwich lamination with a layer of electrically conductive plastics filled with an electrically conductive material. They are not necessarily 100% effective. Although the use of an electrically conductive paint is most popular among them, it is uneconomical because the paint is expensive and it should be applied thick. Incidentally, electromagnetic wave shielding by electroless plating film has recently been fully recognized as effective; however, it is not yet widely spread because of the problems mentioned later.
Printed circuit boards are produced commonly by the subtractive process which consists of applying a photosensitive masking agent to a copper-clad substrate, projecting a reduced image of circuit diagrams to the mask, developing the mask and removing the mask except for the circuit part, thereby causing the copper foil to expose, and finally dissolving and removing the exposed copper foil. This process has the disadvantage of wasting most copper on the substrate and requiring the treatment of copper-containing waste liquid. In addition, because copper foil that can be clad is limited in thickness, this process does not meet the market need for a thinner copper layer.
New processes to obtain a thin copper layer are sputtering and electroless plating. The former is poor in productivity because of necessity for operation in a vacuum, and the latter needs a step of corrosion. An additional but less common process is the full-additive process which is designed to form circuit directly on a substrate by electroless plating. This process also needs a corrosion step and other complicated steps to form circuit.
Conventional electroless plating on plastics necessarily needs a etching step involving treatment with a solution of sulfuric acid-chromic acid mixture. Moreover, using such a dangerous corrosive liquid in large quantities needs measures for environmental protection. Therefore, it cannot be practiced by manufacturers without waste disposal facilities and it needs additional cost for such waste disposal.
Special resin grades for electroless plating have been developed on various plastics. They are modified to make etching easier for improving adhesion between the substrate (plastics) and the plating layer. Such modification, however, partly sacrifices the inherent characteristics of the plastics and also raises the price of the plastics.
In order to solve the above-mentioned problems, there has been developed a process for performing electroless plating without etching on the substrate surface, as disclosed in Japanese Patent Publication No. 33461/1994. This process entails a step of forming a coating film of chitosan or a derivative thereof on the substrate. The disadvantage of this process is that the plating film does not firmly adhere to the substrate and hence needs prime coating to improve adhesion. In fact, the above-cited invention gives an example of using an acrylic-based primer coating but does not give any examples without primer coating.
There are not any other electroless plating processes than mentioned above which do not need a etching step or a coating material filled with palladium (Pd) fine particles as a catalyst, except for that invented by the present inventors mentioned later.
The present inventors have investigated the composition of the resin for coating film and applied for five patents for it. (Japanese Patent Laid-open Nos. 64481/1991, 103771/1992, and 302214/1996; Japanese Patent Publication No. 622531/1995; and Japanese Patent Application No. 256003/1997.)
The Patent Application No. 256003/1997, which is not yet opened, discloses a resin composition for electroless plating and a process for electroless plating using this resin composition. The resin composition is characterized by having in combination (A) a chemical structure with at least one kind of nitrogen-hydrogen bond selected from the group consisting of amino group, imino group, amide linkage, urethane linkage, urea linkage, secondary amine structure, and melamine structure, and (B) a salt-forming functional group. The process consists of applying the above-mentioned resin composition onto an adhesive, non-conductive substrate, thereby forming a coating film thereon, adding a catalyst, and activating the catalyst.
The present inventors' inventions are all designed to perform electroless plating after forming a coating film of resin on the substrate. They premise the use of a new catalyst for electroless plating on plastics such as ABS. This catalyst is a complex salt of PdCl.sub.2 and SnCl.sub.2, with the Pd-containing complex ion being anionic. It permits Pd to adhere easily to a plastic to which Pd does not easily adhere from a simple Pd cationic solution. In addition, it is readily reduced.
The process disclosed in Japanese Patent Application No. 256003/1997 mentioned above is intended to utilize the chemical structure which permits the anionic complex ion to adhere easily. The disadvantage of this process is that plating film is deposited on also uncoated areas unless the substrate is thoroughly rinsed after dipping in the catalyst solution. Therefore, this process presents extreme difficulties in industrially performing electroless plating for electromagnetic wave shielding only on the inside or producing printed circuit boards by full-additive process that employs electroless plating. The latter is almost impossible. In other words, the disclosed process does not match the requirement that the coating film of electroless plating be deposited only on the surface which is coated with a resin composition.
In their investigation into a new process which needs no etching step and permits partial plating, the present inventors tried the dipping of a coating film of the resin composition disclosed in the above-mentioned patent application in a solution of PdCl.sub.2 in place of Pd catalyst of the above-mentioned complex salt in the catalyst imparting step. The solution of PdCl.sub.2 does not adhere readily to ordinary plastics. As a result, it was found that there exists a coating film of resin composition that attracts Pd cations, thereby permitting partial electroless plating. The present inventors further studied the relationship between the capability of partial electroless plating and the chemical structure possessed by the resin composition. It was empirically found that Pd cations adhere to the coating film of a resin composition if the resin composition is composed of a base having the nitrogen-hydrogen bond and a compound with a salt-forming functional group, with the latter being a polybasic acid having a carbon--carbon double bond.
The present inventors assumed that the adhesion of Pd cations is due to ion exchange reaction. This assumption was made based on the fact that the coating film of the resin composition is dipped in a solution of neutral detergent and sodium carbonate in the degreasing step so that the acid radical in the coating film is converted into an Na-salt and then dipped in a PdCl.sub.2 solution in the subsequent catalyst imparting step, in which Pd cations are ion-bonded to the acid radical through ion exchange. In reality, however, such cation exchange should not occur under the strong acidic condition according to common sense of ion exchange reaction because the PdCl.sub.2 solution is kept acidic with hydrochloric acid for its stability. Nevertheless, Pd does adhere to the coating film of the resin composition.
About the phenomenon mentioned above, the present inventors formulated a hypothesis as follows. That is, basic bond (or the nitrogen--hydrogen bond) in the coating film of the resin composition degreased and cleaned in weak alkali condition, is bound with OH.sup.-, and polybasic acid which is another ingredient of the resin composition has double bonds in the main chain consisted of carbon atoms, therefore when the degreased and cleaned film dipped in PdCl.sub.2 solution, neutralization and ion-exchange simultaneously take place in a local region in the molecule, then Cl.sup.- in PdCl.sub.2 solution are adsorbed to basic bond through ion-exchange. The electron of Cl.sup.- push .pi.-electron of the adjoining double bond of polybasic acid out to the next double bond, and then .pi.-electron of the electron accepted double bond is pushed out to next double bond. Such a phenomenon of pushing out .pi.-electron as lined dominos are pushed down one after another by a pushing the first one, is repeated until terminal acid radical of the molecule. Thus un-ionizable weak acid radicals in strong acidic condition, such as carboxyl radical, are ionized, and by these ionized acid radicals, the activated coating film adsorbed Pd cations through ion-exchange. (After this, above mentioned degreasing and cleaning step in weak alkali condition is called activating step.)
The present inventors further contemplated how to prove the hypothesis from the chemical structure of the coating film of the resin composition. Results of series of experiments indicate that double bonds are not necessary in the case of mutual reaction for curing in combination one polymer such as polyethyleneimine or butylated melamine which is dense with basic bonds or nitrogen--hydrogen bonds, with the other polymer which has a low-molecular weight polybasic acid compatible with said polymer. Presumably, this is because the basic bond (or nitrogen--hydrogen bond) comes so close to the acid radical that negative charges are easily transmitted.